This invention relates to a coating color for paper to improve brightness, smoothness, gloss, ink receptivity and the like of the paper. More particularly, this invention relates to a coating color having a low viscosity despite a high solid content, and a method for preparation thereof.
A conventional coating color is prepared by the following steps:
(1) heating a starch slurry containing about 30% by weight of starch such as oxidized starch, aminophosphate esterification starch, hydroxyethyl starch or the like and about 70% by weight of water, over 70.degree. C. for about 20 minutes to impaste within a starch-impasting tank thereby obtaining the impasted starch with a concentration of about 20% to 30% by weight, or enzymatically impasting a starch slurry therein to obtain the enzymatically converted starch with a concentration of about 10% to 35%,
(2) preparing an aqueous pigment-suspension by a mixture of, for example, 70 parts by weight of a pigment of clay with 30 parts by weight of water within a pigment-dispersing tank, and
(3) Mixing the impasted starch slurry with the pigment suspension within a coating-color preparation tank.
The large amount of water necessary to impaste the starch causes a decrease in the solid concentration of the coating color. The removal of excess water in the manufacture of the coated paper is extremely expensive and energy consuming because of necessary drying equipment and the large floor space required for such. The heating of the starch slurry to impaste the starch is also energy consuming. Moreover, these processes are labor consuming.
In order to decrease the large amount of water for impasting the starch, an approach for converting the starch with enzymes in the presence of pigments is disclosed by James P. Casey, "Pulp and Paper, Chemistry & Chemical Technology" Vol. 2, P.1020-1025 (1952). Another approach for converting the starch with enzymes in the presence of the total amount of water to be used in the coating color and adding pigments to the starch after conversion, is also proposed by him in the same article. However, both of those approaches still have the drawback that a large amount of water must be heated and the entire batch of starch and pigments must be cooled before use.